1. Technical Field of the Invention
The present invention relates generally to the field of photonic crystals; and, more particularly, to a photonic crystal drop filter and to a method for tuning the transmission wavelengths of a photonic crystal drop filter.
2. Description of Related Art
Wave division multiplexing is a process that permits the transmission capacity of an optical communications system to be increased. In particular, in a wave division multiplexer (WDM) system, information is transmitted using a plurality of optical carrier signals, each carrier signal having a different optical wavelength. By modulating each carrier signal with a different one of a plurality of information signals, the plurality of information signals can be simultaneously transmitted through a single waveguiding device such as a single optical fiber.
For a WDM system to function properly, the system must have the capability of extracting a carrier signal at a certain wavelength from one waveguide and adding the signal at that wavelength to another waveguide so as to redirect the path through which the extracted carrier signal travels.
FIG. 1 is a block diagram that schematically illustrates components of a WDM communications system. The system is generally designated by reference number 10, and includes a signal source 12 that transmits a plurality of carrier signals at different optical wavelengths through an optical fiber or other waveguiding device 14. The optical fiber 14 is connected to an extraction device 16 that is capable of extracting one or more of the carrier signals carried by the optical fiber 14 and redirecting the extracted signal or signals to another optical fiber or waveguiding device 18. The remaining carrier signals carried by the optical fiber 14 are transmitted through the extraction device 16 to an optical fiber 20 or the like. The carrier signals carried by optical fibers 18 and 20 are then further processed by processing structure not illustrated in FIG. 1.
A practical WDM communications system must be capable of simultaneously transmitting a large number of carrier signals; and, therefore, must be able to carry a large number of light wavelengths. In the future, WDM systems will be required to carry even more carrier signals than today. The number of wavelengths that can be extracted by known extraction devices, however, is finite; and only a distinct set of wavelengths can be derived from any particular extraction device design. Furthermore, he wavelength separation of the carrier signals will be less in future WDM systems; and known extraction devices do not have the resolution that will be required to selectively extract the more closely spaced signals.
For example, drop filters are commonly used in optical communications circuits to extract light of a particular wavelength from one waveguide and direct the extracted light to another waveguide. In effect, a drop filter allows light of one wavelength to be dropped from one path in an optical communications circuit and added to another path in the circuit.
Known drop filters, however, can be designed to extract and redirect only a few distinct, well-separated wavelengths. Accordingly, known drop filters are not fully satisfactory for use as an extraction device in a WDM system that requires the capability of extracting carrier signals carried by light having a large number of different wavelengths.
Photonic crystals (PC) are periodic dielectric structures that can prohibit the propagation of light in certain frequency ranges. More particularly, photonic crystals are structures that have spatially periodic variations in refractive index; and with a sufficiently high refractive index contrast, photonic bandgaps can be opened in the structure""s optical transmission characteristics. (The term xe2x80x9cphotonic bandgapxe2x80x9d as used herein and as is commonly used in the art is a frequency range in which propagation of light through the photonic crystal is prevented. In addition, the term xe2x80x9clightxe2x80x9d as used herein is intended to include radiation throughout the electromagnetic spectrum, and is not limited to visible light.)
A photonic crystal that has spatial periodicity in three dimensions can prevent the propagation of light having a frequency within the crystal""s bandgap in all directions; however, the fabrication of such a structure is often technically challenging. An alternative is to utilize a two-dimensional photonic crystal slab that has a two-dimensional periodic lattice incorporated within it. In a two-dimensional photonic crystal slab, light propagating in the slab is confined in the direction perpendicular to a major surface of the slab via total internal reflection, and light propagating in the slab in directions other than perpendicular to a major surface is controlled by the properties of the photonic crystal slab. A two-dimensional photonic crystal slab has the advantage that it is compatible with the planar technologies of standard semiconductor processing; and, in addition, the planar structure of the slab makes an optical signal in a waveguide created in the slab more easily accessible to interaction. As a result, a two-dimensional photonic crystal slab is susceptible to being used to create active devices.
FIG. 2 is a schematic, perspective view of a two-dimensional photonic crystal slab that is known in the prior art; and is provided to assist in explaining the present invention. The photonic crystal slab is generally designated by reference number 30, and comprises a slab body 32 having a two-dimensional periodic lattice comprising an array of posts 34 therein. As shown in FIG. 2, the posts 34 are oriented parallel to one another and extend through the slab body from top face 36 to bottom face 38 thereof.
The two-dimensional photonic crystal slab 30 can take various forms. For example, the posts 34 can comprise rods formed of a first dielectric material, and the slab body 32 can comprise a body formed of a second dielectric material that differs in dielectric constant from that of the first dielectric material. Alternatively, the posts can comprise holes formed in a slab body of dielectric material; or the posts can comprise rods of dielectric material and the slab body can be air, or another gas, or a vacuum. In addition, the posts can be arranged to define a square array of posts; or they can be arranged in a different manner, such as in a rectangular array or a triangular array.
In a two-dimensional photonic crystal slab such as illustrated in FIG. 2, light propagating in the slab is confined in the direction perpendicular to the slab faces 36 and 38 via total internal reflection. Light propagating in the slab in directions other than perpendicular to the slab faces, however, is controlled by the spatially periodic structure of the slab. In particular, the spatially periodic structure causes a photonic bandgap to be opened in the transmission characteristics of the structure within which the propagation of light through the slab is prevented. Specifically, light propagating in the two-dimensional photonic crystal slab 30 of FIG. 2 in directions other than perpendicular to a slab face and having a frequency within a bandgap of the slab will not propagate through the slab; while light having frequencies outside the bandgap is transmitted through the slab unhindered.
It is known that the introduction of defects in the periodic lattice of a photonic crystal allows the existence of localized electromagnetic states that are trapped at the defect site, and that have resonant frequencies within the bandgap of the surrounding photonic crystal material. By arranging these defects in an appropriate manner, a waveguide can be created in the photonic crystal through which light having frequencies within the bandgap of the photonic crystal (and that thus would normally be prevented from propagating through the photonic crystal) is transmitted through the photonic crystal.
FIG. 3 is a schematic, cross-sectional view that illustrates a two-dimensional photonic crystal slab waveguide apparatus 40 that is known in the prior art. Apparatus 40 comprises a photonic crystal slab 42 comprised of a rectangular array of dielectric rods 44 in air. A region of defects in the photonic crystal slab creates a waveguide 46 through which light having a frequency within the bandgap of the surrounding photonic crystal material can propagate. In the photonic crystal slab of FIG. 3, the region of defects is created by omitting one row of the rods 44. The region of defects can also be created in other ways; for example, by altering the rods in one or more rows such as by removing portions of the rods or by changing the diameter of the rods. The region of defects can extend in a straight line, as shown in FIG. 3, to define a straight waveguide; or the region can be arranged to include a bend, for example, a 90 degree bend, to define a bent waveguide.
Both theoretical and experimental work have demonstrated the efficient guidance of light in a two-dimensional photonic crystal slab waveguide device (see xe2x80x9cDemonstration of Highly Efficient Waveguiding in a Photonic Crystal Slab at the 1.5 xcexcm Wavelengthxe2x80x9d, S. Lin, E. Chow, S. Johnson and J. Joannopoulos, Opt. Lett. 25, pp 1297-1299, 2000). In addition, there has been some investigation into potential applications for interacting with the guided optical modes of the waveguide device. Applications that have previously been discussed include tunable, waveguide dependent devices (see commonly owned, copending U.S. patent application Ser. No. 09/846,856) and channel drop filters (see U.S. Pat. No. 6,130,969).
Photonic crystal devices such as are discussed in U.S. patent application Ser. No. 09/846,856 do not extract and redirect specific wavelengths as are needed in a WDM communications system. In addition, although the tunability of such devices has been demonstrated, the range of tuning of the devices is rather limited. U.S. Pat. No. 6,130,969 discloses a photonic crystal channel drop filter for WDM communications systems; however, the described filter is not tunable. For a drop filter to function effectively in a WDM system, it is desirable that the filter be tunable over a full range of operating frequencies. Thus, existing photonic crystal-based devices are generally not fully satisfactory for use as an extraction device in a WDM system.
Accordingly, there is a need for an extraction device for use in a WDM communications system and for other applications that is capable of extracting and redirecting one or more wavelengths from an optical signal that includes a plurality of wavelengths; and that is continuously tunable so as to be able to extract any selected at least one wavelength from the optical signal.
The present invention provides a tunable, photonic crystal drop filter apparatus that is capable of extracting and redirecting any selected at least one wavelength from an optical signal that includes a plurality of wavelengths.
A photonic crystal drop filter apparatus according to the present invention comprises a photonic crystal having first waveguide for transmitting light having a frequency within a bandgap of the photonic crystal, and a second waveguide. The second waveguide is connected to the first waveguide by a resonant cavity for extracting at least one wavelength of the light transmitted by the first waveguide and redirecting the extracted light to the second waveguide. The apparatus also includes a tuning member for controlling the at least one wavelength of the light extracted from the first waveguide.
The resonant cavity modifies the transmission characteristics of the first waveguide by creating one or more transmission zeros, that comprise narrow frequency ranges within the bandgap of the photonic crystal material at which light that is otherwise capable of being transmitted through the first waveguide is prevented from propagating through the first waveguide, i.e., is xe2x80x9cfilteredxe2x80x9d out of, the first waveguide. By connecting a second waveguide to the first waveguide through the resonant cavity, the light that is prevented from propagating through the first waveguide is redirected to the second waveguide. As a result, a drop filter is provided that is capable of removing light of one or more wavelengths from the first waveguide and redirecting the removed light to the second waveguide.
According to one embodiment of the invention, the tuning member extends into the second waveguide and is movable within the second waveguide. The wavelengths of the light that can be extracted from the first waveguide is a function of the position of the dielectric tuning member with respect to the resonant chamber, and by adjusting the position of the dielectric tuning member, the extracted wavelengths can be precisely controlled.
According to another embodiment of the invention, a moving device is connected to the dielectric tuning member to move the dielectric tuning member to desired positions in the second waveguide so as to provide the apparatus with the capability of being continuously tuned. Preferably, the moving device comprises a micro-mover capable of moving the dielectric tuning member by very precise amounts so as to permit the wavelengths of the extracted light to be very precisely controlled.
A photonic crystal drop filter apparatus according to the present invention is capable of precisely controlling the wavelengths of light extracted from an optical signal and can be continuously tuned to extract any selected wavelengths within a wide range of wavelengths. The apparatus is, accordingly, particularly suitable for use as an extraction device in WDM communications systems and in other applications that require the extraction of one or more wavelengths of light from a signal that includes a plurality of wavelengths.
Yet further advantages and specific features of the present invention will become apparent hereinafter in conjunction with the following detailed description of embodiments of the invention.